Many circuits employ power amplifiers, such as RF power amplifiers in handheld communication devices and other wireless devices. Power amplifiers are often controlled by adjusting a supply voltage provided to the power amplifiers. A low drop out (LDO) linear amplifier is routinely used to generate the supply voltage for a power amplifier.
Linear amplifiers can be very efficient when used to generate the supply voltage for a power amplifier. For example, when the supply voltage is very close to the power supply of a linear amplifier (often a battery voltage), the linear amplifier is typically very efficient. However, when the supply voltage is far below the battery voltage, the linear amplifier may be very inefficient.
Linear amplifiers can also be designed to have extremely low noise and very high bandwidth. The low noise property is often important because there can be strict linearity requirements placed on power amplifiers and any noise in the supply voltage can translate into power amplifier output noise. The high bandwidth property is also often important because there can be spectral mask and time mask requirements in different communication systems, such as when the supply voltage needs to be fully ramped up or down rapidly and when the supply voltage needs to maintain its modulation dynamics.
Switching converters can provide high efficiency, but they typically have limited bandwidth. This may be due to several reasons, such as high switching losses at high switching frequencies, low resonant frequencies of external inductor-capacitor components, and control difficulties. Also, switching converters typically produce switching noise at the desired switching frequencies. As a result, typical switching converters are inappropriate for use in generating a supply voltage requiring envelope tracking or short time masks for a power amplifier.
One solution proposed to resolve these problems includes a switcher that provides a supply voltage for a linear amplifier, which in turn provides a supply voltage and envelope tracking for the power amplifier. For example, the switcher may generate the peak power needed by the linear amplifier, and the linear amplifier may fine tune its output to make sure its output tracks a reference input voltage. In this way, the supply voltage for the power amplifier may be generated in a more efficient manner by combining the properties of switching converters and linear amplifiers.
However, the linear amplifier for this solution has to pass the full load current to the power amplifier. Consequently, the linear amplifier has a non-negligible loss associated with its operation. For standards demanding a large amount of bandwidth, such as 20 MHZ for LTE, the high bandwidth may be difficult to achieve without the linear amplifier consuming a large quiescent current due to the large amount of current being provided to the power amplifier. Thus, this system may be highly inefficient in this scenario. In addition, for applications with a high peak/average ratio, such as OFDM for WiMax, a high dropout associated with the linear amplifier may result in a much less efficient linear amplifier.